A known multistring inverter is the product “Sunny TriPower” by the Applicant. The plurality of DC-to-DC converters of a multistring inverter make it possible to operate the strings connected via said DC-to-DC converters in each case independently of one another at their maximum power point (MPP), i.e. at the operating voltage at which the maximum electric power is generated by the strings. In this case, the DC-to-DC converters are typically boost converters, which step up the operating voltage of the individual strings to the DC-link voltage of a common DC link, which is an input DC link of the DC-to-AC converter. In the case of the product “Sunny TriPower”, the deration of the power of a string always takes place in the direction of its off-load voltage. In this case, first the string with the greatest voltage difference between its operating voltage and the DC-link voltage is relieved of load in the event of deration of the power of the DC-to-AC converter, so that its operating voltage increases in the direction of its off-load voltage. Only when this string has been completely relieved of load or when the voltage difference is equal to the voltage difference for a second string is the second string also relieved of load. As a result, minimization of the power losses in the individual DC-to-DC converters in the form of boost converters is achieved, as well as adjustment of their power losses. As already explained, the shift in the operating voltage in the case of inverters of this type always takes place in the direction of the off-load voltage of the individual strings. Therefore, when the installation is dimensioned, it is necessary to ensure that the maximum off-load voltage of the strings does not exceed the maximum permissible DC-link voltage.
In a known version of the multistring inverter “Sunnyboy” by the Applicant, the possibility is provided to the installation operator to extend the range of the possible operating voltages of the strings. In order that the DC-link voltage does not exceed its maximum permissible value nevertheless, in the case of deration of the power of the DC-to-AC converter the operating voltage is shifted in the direction of a short circuit of the respective string, i.e. from the MPP towards lower voltages. This makes it possible to design the strings in such a way that the maximum MPP voltage is approximately equal to the maximum permissible DC-link voltage. Specifically, during control of this known multistring inverter, even prior to the connection of the strings to form a common DC link, it is detected whether a voltage-extending string, i.e. a string with an off-load voltage above the maximum permissible DC-link voltage, is present. Run up of the MPP of such a string is then performed from the range of low operating voltages of the string; and, during deration of the power of the DC-to-AC converter, such a string is operated at reduced operating voltages. Other strings which are not voltage-extending are shifted, during the deration, towards higher operating voltages, i.e. towards their off-load voltage, on the other hand, because a better, in particular quicker deration capacity of the power thereof is provided in this direction owing to the profile of the characteristics of the strings. If a string which initially is not voltage-extending becomes voltage-extending nevertheless owing to changed operating conditions, i.e. for example during deration of its power by increasing its operating voltage in the direction of its off-load voltage, an operating voltage above the permissible DC-link voltage is achieved, the operating voltage of the affected string and possibly substrings connected in parallel to said string and connected to the same DC-to-DC converter is short-circuited, as a result of which a further increase in the DC-link voltage is prevented. Then, the power fed in by the multistring inverter needs to be run up again. As a result, losses during feeding occur since the currently permissible power of the DC-to-AC converter can only be fed again after a few seconds in the case of renewed run up owing to a maximum increase in the power ramp. In general, it is of interest to exhaust the power which can presently be fed by the DC-to-AC converter as much as possible.
US 2011/0101784 A1 discloses a hybrid wind and solar inverter. In this case, power is fed to an electrical grid from alternative voltage sources. Depending on the demand for electric power on the grid, the electric power from the individual voltage sources is connected to the grid or buffer-stored.
EP 2 284 382 A2 discloses an energy supply system, in which electrical energy which is provided by various voltage sources on a power bus is consumed locally or is buffer-stored in the form of heat.
EP 2 104 200 A1 discloses a method for actuating a multistring inverter for photovoltaic systems, which multistring inverter has a separate DC-to-DC converter on the input side for each string. In order to improve efficiency, one or more electrical variables, to be precise input current, input voltage and/or input power, are measured at each DC-to-DC converter, and at least one of the DC-to-DC converters changes its operating state in the event that a limit value and/or a window is exceeded, depending on this measurement, in such a way that its power losses are reduced. For example, the DC-to-DC converter can be disconnected when a boosting of the operating voltage of the string connected thereto, initiated by it, is no longer required because the operating voltage reaches the DC-link voltage.
WO 2012/017068 A2 discloses a method for detecting a feed-in energy quantity, which has been potentially possible but has not actually been fed in within a time period, of a photovoltaic system comprising one or more inverters for feeding electrical energy from one or more photovoltaic generators into an AC grid. Said method is intended in particular for detecting the feed-in power which has been potentially possible during deration. In order to enable this detection, the inverter(s) is/are operated differently during the deration, wherein the different operating modes comprise operation at the MPP or at least for detecting the characteristic of the strings connected to the respective inverter. Specifically, one or more inverters can each be operated at the MPP, and the power of the other inverters can be reduced more significantly in order to achieve the desired deration than if the deration were to be distributed uniformly among all of the inverters. In another specific embodiment of the known method, the characteristics of the string(s) connected to the inverter is sampled regionally or completely, and the thus varying power of the inverter is buffer-stored or compensated for at different times, so that the required power reduction, when averaged over time, is achieved. The demands placed on the deration of the electric power and in particular on the provision of negative deration power do not permit short-term overshooting with the electric power provided by an inverter either, however. In order to feed a constant electric power despite the variable electric power of the inverter, WO 2012/017068 A2 proposes buffer-storing the difference between the different electric power and the constant electric power or converting it into other forms of energy. For this purpose, however, additional devices are then required.